The invention relates to a method for heating fiber reinforced thermoplastics in contact heating means and also to an apparatus for carrying out the method.
Fiber reinforced, for example glass mat reinforced thermoplastics, as semi-finished products are usually in the form of sheets. To process them by pressing, these sheets have to be heated to a temperature which is usually about 60-80 degrees above the melting point of the thermoplastics. According to the prior art, this is usually accomplished in infrared ovens or hot air ovens, i.e. the heat is transferred either by radiation or by forced convection.
A disadvantage of heating by radiation is local overheating of fibers detaching themselves from the compact surface and of matrix adhering to them. Since the radiation source is usually several 100 degrees hotter than the fiber reinforced thermoplastic to be heated and since the removal of heat from a fluffy structure in the direction of its base is poor, the tips of the fibers become very hot before the entire sheet has heated through. In this case, the adhering thermoplastic is damaged, under some circumstances even vaporizes, which is disadvantageous for the quality of the parts to be pressed. Contact of the hot parts with the ambient air also contributes to the damage by oxidation.
When heating is carried out in circulating air ovens, the problem of local overheating is greatly reduced, but by the same token the supply of oxygen is increased and, in particular in the case of small parts, there arises the problem that they are "blown" by over-intensive air flow, which, in the interests of a troublefree production sequence, must be avoided under all circumstances by limiting the air speed, but consequently also the heat transfer. Since, moreover, for the purpose of avoiding thermal damage to the product (the thermoplastic), the air temperature is also limited to a value not much higher than the thermoplastic temperature to be reached, generally long heating-up times of the order of 10 to 15 minutes are required. In the case of continuous production, this means that the circulating air oven must be large enough for a plurality of blanks to fit in it at the same time for heating up.
Both methods share the disadvantage of poor thermal efficiency, i.e. the ratio of energy supplied to energy actually taken up by the fiber reinforced thermoplastic is unsatisfactory.
To avoid these problems, it has also been attempted to transfer the required heat to the fiber reinforced thermoplastics by heat conduction. This is achieved by bringing the fiber reinforced thermoplastic to be heated between 2 heating plates, of which at least one is movably arranged and which are pressed together for heating. Problematical in this case is the removal of the heated fiber reinforced thermoplastic from such an apparatus, since it adheres intensively to the heating surfaces. To solve this problem of adhesion, in the case of contact heating ovens it was attempted not to bring the thermoplastic directly into contact with the heating plates, but between 2 bands which are as thin as possible and conduct heat as well as possible. In the case of industrial-scale configurations, these are designed as circulating bands. In this case as well, the thermoplastic can be detached from the bands only with great difficulty. These bands consist, for example, of a glass fabric, which is coated or impregnated with silicones or fluoropolymers, such as for example polytetrafluoroethylene (PTFE). They may also consist of metallic materials, for example steel.
To detach the heated thermoplastic from the bands, it is proposed by J. Dykhoff and W. Michaeli (Kunststoff-Journal 5-92, pages 16-22) to quench the bands. The superficially solidified thermoplastic can then actually be detached from the bands. The disadvantage of this method in terms of energy is obvious: an amount of heat equivalent to that which is removed from the thermoplastic during quenching must subsequently be additionally supplied to the thermoplastic again, or be stored as an elevated temperature, for example it has to be supplied to the thermoplastic again after quenching by an additional heating means, for example a radiation heating means. In this case, there are further major disadvantages in addition to the high energy losses. During quenching, the heat must be removed uniformly and rapidly over the entire surface area of the thermoplastic in order to ensure uniform flowing behavior of the fiber reinforced thermoplastic. However, rapidly and uniformly are mutually contradictory in practice, with the result that the temperature uniformity of heated thermoplastic removed from a contact oven in this way is unsatisfactory. A further problem is that the quenched surface leads to an increased visibility of the insert in the pressed component.